1. Field of the Invention
The present invention relates generally to low dropout ("LDO") voltage regulators. More particularly, the present invention relates to improvements in LDO voltage regulators that use a "follower" connected pass element to address the problems of output over-voltage conditions and instability at low output currents.
2. State of the Art
The function of a voltage regulator is to take a varying input voltage supply and generate a stable output voltage. The efficiency of modern power supply systems, particularly battery powered supply systems, is directly related to the amount of power dissipated in the voltage regulator. Minimizing the power consumption is a key parameter in regulator design. The primary method for reducing power consumption is to reduce the voltage drop across the linear regulator. The lowest voltage drop the regulator can tolerate before loss of regulation occurs is called the "dropout voltage" and a low dropout voltage is very desirable. For battery powered systems, power is limited and efficiency is of key importance. Thus, the design of an efficient system that utilizes linear regulation necessarily includes a low dropout ("LDO") voltage regulator.
As shown in FIG. 1, a linear voltage regulator 2 conventionally includes an amplifier 4 which compares the output of a voltage reference 6 to a sample of an output voltage supplied by feedback elements 8. The output of the amplifier 4 is coupled to a control terminal 10 of a pass element 12 which serves to "pass" current from the unregulated input terminal 14 of the voltage regulator 2, to the regulated output terminal 16 of the voltage regulator 2. The feedback control loop 18 formed by the amplifier 4, pass element 12 and feedback elements 8 acts to force the control terminal 10 of the pass element 12 to a dynamic value that maintains a regulated voltage at the output terminal 16 of the voltage regulator 2.
The pass element 12 may be used in a common source/emitter configuration or a common drain/collector follower configuration. A voltage follower configuration has the advantages of not requiring a large output capacitance, having a better response time for transient signals, and providing greater immunity to output capacitor characteristics. Greater immunity to output capacitor characteristics is a significant advantage in low power LDO voltage regulators.
In either configuration, however, the pass element 12 functions as a "unipolar" element in conventional designs. A "unipolar" element, as used herein, is one which sources current to the load, but does not sink current from the load. In other words, a unipolar element can supply needed electrical charge to a load, but cannot remove excess electrical charge from the load. A load conventionally includes at least one large output capacitor 20. A linear voltage regulator 2 configured with a unipolar output stage, however, experiences two common problems: an output over-voltage or "hiccup," and instability at output current levels below a required minimum output current value.
First, when the load current required at the output terminal 16 of the voltage regulator 2 rapidly changes from a large value (e.g. near a maximum rated output) to a relatively small value (e.g. near zero), more current than is necessary may be supplied to the output terminal 16 until the feedback loop 18 regains control due a finite response time associated with the feedback control loop 18. The excess charge is stored on the output capacitor 20 and results in an output voltage higher than the desired regulation voltage. The increased voltage at the output terminal 16 causes the feedback control loop 18 to attempt to reduce the output voltage by reducing or stopping the current passing through the pass element 12. Even with the pass element 12 turned off, however, the output voltage remains high for a time because the feedback control loop 18 cannot remove the excess charge from the output capacitor 20. As a result, the feedback control loop locks-up, and the output terminal 16 remains in an over-voltage condition until the excess charge drains off of the output capacitor 20. This transient overvoltage is sometimes called a "hiccup."
In applications where the load current is small, this discharge process may take a relatively long time. Although the voltage regulator 2 includes a discharge path through the feedback elements 8, the amount of discharge through the feedback elements 8 is typically insignificant because the feedback elements 8 conventionally comprise large valued resistive elements. While the feedback control loop 18 is locked up and, therefore, unable to regulate, the voltage on the output capacitor 20 may be in a range that is harmful to the load circuitry and, therefore, have serious consequences.
The "hiccup" condition may also be further exacerbated when the excess charge is discharged from the output capacitor 20 and the voltage regulator 2 again begins to pass current through the pass element 12. As the feedback control loop 18 begins to respond to the need for more charge on the output, the pass element 12 is turned back "ON" to allow current to pass. This rapid change in current may result in another "hiccup" from the pass element 12 again passing too much current before the feedback control loop 18 has time to respond. With each subsequent "hiccup," the feedback control loop 18 locks up and takes time to recover during which it cannot properly regulate the output voltage. Each subsequent "hiccup" decreases in magnitude until the feedback control loop 18 no longer locks up. In other words, the feedback control loop 18 oscillates between locking-up and being in control of the pass element 12 for a time following an initial "hiccup."
Second, the stability problem occurs under low or no-load conditions where the only current passing through the pass element 12 is due to the current passing to ground through the feedback elements 8. As stated previously, because the feedback elements 8 conventionally include large valued resistive elements, this current is very small compared to a current for a load at the output terminal 16, and is typically below the minimum output current requirements of the pass element 12. This small current in the relatively large pass element 12 causes low transconductance (g.sub.m) due to low current density therein, decreases loop gain and increases output impedance, potentially causing an unstable condition. An unstable condition results from the voltage regulator failing to regulate the output voltage which may cause the output voltage to oscillate undesirably until the specified minimum output current again flows through the pass element 12. This problem is more pronounced with pass elements 12 implemented as "followers" configured as a common drain or a common collector amplifier.
Early linear voltage regulators used a pass clement 12 which was an NPN transistor in an emitter follower configuration. These early voltage regulators did not require LDO characteristics, and conventionally did not have load currents which rapidly transitioned between high and low values during periods where tight output voltage regulation was required. Thus, the above described "hiccup" and minimum current problems were not significant. However, as dropout became more important (i.e. with battery powered systems), LDO voltage regulators 22 were introduced which used the pass element 24 in the common emitter and, later, common source configurations (FIG. 2). FIG. 2 illustrates a conventional LDO voltage regulator 2 implementation of the circuit shown in FIG. 1. For the LDO voltage regulator 22, the reference voltage 26 (which may be provided by a bandgap reference or any other voltage reference generator known in the art) is applied to the inverting terminal 28 of the error amplifier 30. The error amplifier 30 compares the voltage reference 26 at the inverting terminal 28 to the output voltage sample provided by the feedback network 32, and controls the gate/base of a PMOS/PNP pass element 24 coupled between the input 34 and output 36 terminals of the voltage regulator 22.
As battery powered or power managed applications became more prevalent in the market, loads that switch from full current to zero or nearly zero current became more common and the hiccup problem became more of a concern. A first example of an approach to addressing the hiccup problem is described in U.S. Pat. No. 5,864,227 to Borden et al. (Jan. 26, 1999), an embodiment of which is shown in FIG. 3. In addition to the conventional elements used in prior art voltage regulator circuits, the Borden et al. approach uses a voltage regulator 38 having a "pull-down" circuit 40 comprising a secondary reference voltage 42, a comparator 44, and a pull-down transistor 46. When the comparator 44 senses that the voltage at the control terminal 48 of the pass element 50 is approximately equal to that of the secondary reference voltage 42, it turns the pull-down transistor 46 "ON" to draw current from the output capacitor 52 until the feedback control loop recovers. The Borden et al. approach may be used for LDO regulators using a pass element 50 configured in the common source or common emitter configurations.
The voltage regulator shown in FIG. 3, however, utilizes a more digital than linear approach to controlling the voltage at the output terminal 51. In an over-voltage or hiccup condition, a low impedance or a current source "load" is introduced through the pull-down transistor 46 until the over-voltage is discharged. This approach requires the feedback loop to be out of control before it can function, and, therefore, has an attendant response and recovery period for each lock-up condition. Furthermore, the voltage regulator circuit 38 of FIG. 3 requires at least one additional comparator 44 to implement, and, therefore, uses more chip area, and still fails to address the problem of instability at the minimum output current. Additionally, for the voltage regulator 38 of FIG. 3, the pass element 50 must be configured as a common source or common collector amplifier and, thus, cannot achieve the advantages of a voltage follower configuration.
A second non-prior art example of an approach to addressing the hiccup problem is fully described in the commonly assigned co-pending patent application entitled OVERVOLTAGE SENSING AND CORRECTION CIRCUITRY AND METHOD FOR LOW DROPOUT VOLTAGE REGULATOR by Tony Larson and David Heisley, U.S. patent application Ser. No. 09/560376 to Larson et al. (filed Apr. 28, 2000). An embodiment of the Larson et al. approach is shown in FIG. 4. The Larson et al. approach to resolving the hiccup problem, like the Borden et al. approach, uses a voltage regulator 56 having a pull-down circuit 58. Unlike the Borden et al. approach, however, the Larson et al. approach does not require a secondary reference voltage. The Larson et al. approach uses the primary reference voltage 60 as a reference for determining when the comparator 62 will turn the pull-down transistor 64 "ON" and "OFF." Thus, when the inputs to the error amplifier 66 are such that the pass element 68 is turned "ON," the oppositely configured inputs to the comparator 62 will be such that the pull-down transistor is turned "OFF." Conversely, when the inputs to the error amplifier 66 are such that the pass element 68 is turned "OFF," the oppositely configured inputs to the comparator 62 will be such that the pull-down transistor is turned "ON" to sink the excess charge on the output capacitor 70 until the voltage at the output terminal 72 is within regulation. The Larson et al. approach may be used for LDO regulators using a pass element 68 configured in either the common source or common drain configurations.
Similar to the Borden et al. approach, however, the Larson et al. approach implements a comparator 62 to initiate a low impedance load under over-voltage conditions until the over-voltage is discharged and is, thus, locked-up during over-voltage periods. Also similar to the Borden et al. approach, the Larson et al. approach requires at least one additional comparator 62 to implement.
A third example of an approach to addressing the hiccup problem is described in U.S. Pat. No. 5,608,312 to Wallace (Mar. 4, 1997), an embodiment of which is shown in FIG. 5. The Wallace approach uses a pair of differential amplifiers 74 and 76 to control a pair of common source pass elements 78 and 80 in a push-pull configuration. The inverting inputs 82 and 84 are coupled to a reference voltage 86, and the non-inverting inputs 88 and 90 are connected to an output node 94. When the first pass device 78 is turned "OFF," the second pass device 80 is turned "ON" to pass excess charge from the output capacitor 92 to ground. The Wallace approach is appropriate for use in SCSI terminator regulators that utilize complementary pass elements 78 and 80 in the common source or common emitter configurations.
The voltage regulator circuit shown in FIG. 5, however, cannot achieve the advantages of a voltage follower configuration because the pass elements 78 and 80 must be configured as common source or common emitter amplifiers. Furthermore, although the voltage regulator circuit of FIG. 5 can help stability under low load or no load conditions, because at least one pass element is on during both source and sink, it has difficulty controlling the bias current in each of the pass elements.
Therefore, it is desirable to have a voltage regulator which avoids the over-voltage or hiccup problem in addition to overcoming the problem of instability at low output currents.